1. Field of the Invention
The present invention relates to environmental and metric sensors and more particularly to environmental and metric sensors wherein light signals in a multiplicity of optical fibers are modified, with the utilization of environmental and positional sensitive elements, in accordance with environmental and positional variations.
2. Description of the Prior Art
Fiber optic transducers of the prior art generally utilize a membrane, movable with ambient variations. This membrane may be coupled to at least one of two optical fibers, having opposing end faces across a gap that are movable therewith to effectuate a variation of the coupling of light propagating across the gap between the end faces of the optical fibers. One such device is disclosed in U.S. patent application Ser. No. 71,893, now U.S. Pat. No. 4,300,813, filed Sept. 4, 1979 by R. L. Gravel and assigned to the assignee of the present invention. This device includes two optical fibers, each cut to have end faces substantially perpendicular to an axis and positioned with a small gap between the end faces of the fibers. One fiber is mounted to maintain its end face stationary, while the other is cantilevered and coupled to a membrane movable with variations in ambient pressure, thus effectuating a relative movement of the optical fibers and a variation of the coefficient of coupling optical energy therebetween.
A fiber optic transducer which provides improved sensitivity over that of the Gravel device is disclosed by D. H. McMahon in U.S. patent application Ser. No. 133,171, now U.S. Pat. No. 4,293,188, filed Mar. 19, 1980 and assigned to the assignee of the present invention. As in the Gravel device, the transducer disclosed by McMahon comprises a first optical fiber disposed with its end face stationary while the second optical fiber is coupled to a membrane movable with variations in ambient pressure and disposed so that its free end may be laterally displaced from the axis of the first optical fiber in accordance with the movement of the membrane. The sensitivity of the transducer is enhanced through the use of large core, large numerical aperture, multimode optical fibers. The opposed faces of the cooperating fibers are equipped with or used to illuminate regular arrays of equally spaced opaque, absorptive, or reflective grating systems providing intensity or phase modulation of the propagating light energy in proportion to the optical fiber deflection. Another transducer of the prior art for converting variations in ambient pressure to intensity modulated optical signals is disclosed in U.S. patent application Ser. No. 63,504, now abandoned, filed Aug. 3, 1979 by D. H. McMahon and assigned to the assignee of the present invention. This transducer is provided by cutting and polishing the ends of two optical fibers, having equal indices of refraction, at predetermined angles with respect to their axes and positioning the angled end faces in parallel relationship a distance apart that is less than the wavelength of the light propagating within the input fiber, thus creating a gap between the end faces wherein a material with a second refractive index is contained. The angled end faces are held in position by two plates through which the fibers extend, and are coupled to pressure sensing elements of the system. Variations in pressure cause the distance between the end faces of the fibers to vary producing variations in the optical signal energy coupled between the optical fibers thus creating an amplitude modulated light beam at the output port of the system.
The transducers of the prior art, described above, are capable of converting motions in the order of 10.sup.-10 cm to measurable and identifiable optical signal modulations. These, however, are absolute measurements, the accuracy of which is a function of the optical source stability. Additionally, to achieve the above-quoted accuracy, difficult and time consuming procedures must be employed to calibrate the system.